Erythrocyte (NA ion plus K ion)ATPase activity has been measured in Caucasi and Pima Indian subjects. Obese Pima subjects have a significantly altered Mg ion dependence for enzyme activity compared to thin Pima or Caucasians. Additional studies will determine the importance of this lesion as a genetic factor which promotes obesity. For this purpose, the activity will also be measured in diploid fibroblasts from thin and obese Pima Indian and Caucasian subjects.